Aerosol Detection

ABSTRACT

Aerosol detection apparatus comprises an aircraft having a dielectric member, such as a window ( 10 ), comprised in the body ( 12 ) thereof such that a surface of the dielectric member forms part of the exterior surface of the aircraft. Detection means ( 16 ), such as a static monitor is located on the inside of the aircraft and arranged to detect an electric field resulting from polarisation of the dielectric member. The output of the static monitor, or the rate of change thereof, correlates closely to particle density as the aircraft is flown though an aerosol, such as a volcanic ash cloud. The apparatus is simple and relatively inexpensive, and may comprise any general purpose aircraft. Aerosol particles may be detected and mapped using apparatus of the invention more easily and quickly than by use of devices such as optical spectrometers mounted on dedicated research aircraft, or static monitors mounted on the exterior of an aircraft.

The invention relates to apparatus and methods for aerosol detection,and particularly to the detection of solid particles, such as particlesof ash, dust, ice, snow, rain or pollution, in the atmosphere.

Airborne particulates are typically detected and analysed over largeregions of the atmosphere by means of complex particle-sensinginstrumentation mounted on aircraft. For example, an aerosolspectrometer may be mounted on an aircraft, and the aircraft may then beflown through the atmosphere with air being drawn through thespectrometer by a vacuum pump. Certain commercially available aerosolspectrometers, such as the Model 1.129 Sky-OPC manufactured by GrimmAerosol Technik GmbH & Co KG, are specifically designed for atmosphericresearch, and allow data relating to particle size and particle densityto be recorded on an integrated data storage card as a function of timeand/or the position of an aircraft on which the spectrometer is mounted.However such devices are complex and expensive. They require significanttime and effort to be fitted to aircraft. Particle analysis in suchdevices is typically carried out by means of optical scatteringmeasurements in which light from a laser or LED is scattered by theparticles, and the presence, size and density of the particles isinferred from measurements of scattered light. This involves a complexarrangement of detectors and complex computer processing resources.Furthermore, dedicated research aircraft are generally required becauseof aircraft safety certification regulations. This means that commercialaircraft which fly through a given region of the atmosphere, and whichcould potentially gather data on airborne particulates as a function ofposition in the region, are not able to be exploited to gather suchdata.

A first aspect of the present invention provides aerosol detectionapparatus comprising an aircraft having a dielectric member comprised inthe body thereof such that a surface of the dielectric member forms partof the exterior surface of the aircraft, and detection means located inthe interior of the aircraft and arranged to detect an electric fieldresulting from polarisation of the dielectric member.

When the aircraft is flown through airspace containing airborneparticles, such as dust or ash for example, the dielectric memberbecomes polarised so that induced charge appears on the surface of thedielectric member opposite to that forming part of the exterior surfaceof the aircraft. By detecting an electric field within the aircraftresulting from this induced charge, the presence of particles in theatmosphere may be inferred. Since the bodies of aircraft are typicallymetallic, it has previously been assumed that if an aircraft were tobecome charged for any reason, no electric field would be present in theinterior of the aircraft because the aircraft would behave as a Faradaycage. For example, aircraft charging has been noted previously, butdetected by placing instruments on the exterior of an aircraft (R. C.Roberts & G. W. Brock, Journal of Meteorology, volume 2 (1945), pp205-213; R. C. Waddel, R. C. Drutowski & W. N. Blatt, Proceedings of theInstitute of Radio Engineers, volume 34 (1946), pp 161-166). Thepolarisation of the dielectric member may take place by one or more of anumber of different mechanisms. Aerosol particles which are alreadycharged can transfer their charge to the surface of the dielectricmember forming part of the exterior of the aircraft, as the aircraft isflown through the aerosol. Uncharged aerosol particles may also causecharging of that surface by a frictional mechanism. Also, other parts ofthe exterior surface of the aircraft may become charged during flightthrough an aerosol, producing an electric field which polarises thedielectric member, or assists in the polarisation of the dielectricmember.

The dielectric member may be a window of the aircraft, in which case anygeneral purpose aircraft may be used. In other words no specialdielectric member is required to be retro-fitted to an aircraft, or usedin the construction of a new aircraft, in order to implement theinvention. For example, a window of BAe ‘146’ aircraft comprises anexternal layer of acrylic, which serves well as a dielectric member.

The detection means may be a static monitor mounted within the aircraft.An electro-static voltmeter, such as electro-mechanical field millinstrument, may be used. A suitable electro-mechanical field mill is theJCI 140 static monitor manufactured by Chilworth Technology Ltd ofSouthampton, U.K.

The apparatus may further comprise alarm means arranged to monitor anoutput of the static monitor and generate and alarm if said outputexceeds a pre-determined threshold. The alarm could be a visual and/oraudible signal for the attention of one or members of the aircrew.Additionally or alternatively it may be a control signal to the aircraftcontrol systems causing the aircraft to move out of a certain region ofairspace.

Preferably the apparatus further comprises a data acquisition systemarranged to record the output of the static monitor, or the rate ofchange of the output of the static monitor, as a function of position ofthe aircraft. The electric field resulting from accumulated charge onthe surface of the dielectric as the aircraft is flown through airspacecontaining particles indicates the presence of an aerosol. Recording theoutput of the static monitor (or its rate of change) as a function ofposition allows the presence of aerosol particles to be mapped.

Aircraft position may be obtained in a number of ways. For example whenflying at constant velocity, total elapsed flight time is a measure ofaircraft position. However in order to give accurate and detailedpositional information (latitude, longitude and altitude), and hencemore accurate and detailed mapping of the presence of particles in theatmosphere, the apparatus preferably further includes a globalpositioning system (GPS) arranged to output positional information forthe aircraft to the data acquisition system.

The value of the electric field resulting polarisation of the dielectricmember (and hence the output of the static monitor) may be a function ofaerosol particle density. The rate of change of the electric field (andhence the rate of change of the output of the static monitor) may alsobe a function of aerosol particle density. The apparatus thereforepreferably further includes processing means arranged to convertrecorded values of the output of the static monitor, or as the case maybe recorded values of the rate of change of the output of the staticmonitor, to values of aerosol particle density on the basis of anassumed functional form for aerosol particle density as a function ofthe output, or as the case may be the rate of change of the output, ofthe static monitor. This allows aerosol particle density to be mapped,i.e. aerosol particle density to be determined as a function of aircraftposition. The assumed functional form may be linear or non-lineardepending on the circumstances, for example the type of particle in theatmosphere.

As an alternative to processing means arranged to convert values of theoutput of the static monitor, or the rate of change of output of thestatic monitor, stored in a data acquisition system to values of aerosolparticle density, the apparatus may instead comprise processing meansarranged to convert the output of the static monitor, or the rate ofchange of the output of the static monitor, directly (i.e. in real time)to a values of aerosol particle density on the basis of an assumedfunctional form for aerosol particle as a function of the output of thestatic monitor, or as the case may be the rate of change of the outputof the static monitor. In this case the apparatus may further comprise adata acquisition system arranged to record values of aerosol particledensity output by the processing means as a function of the position ofthe aircraft, so that the data acquisition system stores a mapping ofaerosol particle density. In this case also, preferably the apparatusfurther comprises a global positioning system (GPS) arranged to outputpositional information for the aircraft to the data acquisition systemfor the reasons given above.

A second aspect of the invention provides a method of detectingparticles in an aerosol comprising the step of causing an apparatus ofthe invention to pass through a region of the atmosphere containing theparticles.

Embodiments of the invention are described below, by way of exampleonly, and with reference to the accompanying drawings in which:

FIG. 1 shows a portion of a first example apparatus of the invention;

FIG. 2 shows a dielectric member of the FIG. 1 portion in more detail;

FIG. 3 shows a portion of a second example apparatus of the invention;

FIG. 4 shows traces of aerosol particle density obtained using anephelometer and of the output of a static monitor comprised inapparatus of the invention;

FIG. 5 shows traces of aerosol particle density obtained using anoptical spectrometer and of the output of a static monitor comprised inapparatus of the invention; and

FIG. 6 shows traces of aerosol mass density obtained using dedicatedinstrumentation and of the rate of change of the output of a staticmonitor comprised in apparatus of the invention.

FIG. 1 shows a portion of a first example apparatus of the invention,the apparatus comprising a BAe ‘146’ aircraft having metallic fuselage12 having a window 10, an outer surface of which forms part of theexterior of the aircraft. An instrument package 20 is mounted on theinterior of the aircraft, the instrument package 20 comprising anelectro-mechanical field mill sensor 16 (e.g. model JCI 140 staticmonitor manufactured by Chilworth Technology Ltd, Southampton , U.K.).The output of the sensor 16 is coupled to a data acquisition system 18which is arranged to record the output of the sensor 18 at regularintervals, each value of the output of the sensor 16 being recordedtogether with the position of the aircraft at the time the output isrecorded. A global positioning system (GPS) unit 22 is arranged tosupply positional information relating to the aircraft to the dataacquisition system 18. A processor 24 coupled to the data acquisitionsystem 18 is arranged to process information stored in the dataacquisition system 18.

FIG. 2 shows the window 10 of the aircraft in more detail. The window 10is made up of two structural layers 10A, 10B of acrylic, with a thirdinternal layer 10C of acrylic which acts as a thin scratch panel. Theouter surface of the layer 10A forms part of the exterior of theaircraft.

In use of the apparatus, the aircraft is flown through a region of theatmosphere containing particles of dust, ash, pollution etc, in otherwords a region of the atmosphere which is an aerosol. Aerosol particleswhich are charged and which impinge on the outer surface of the window10 can transfer their charge to the outer surface of the window 10. Inaddition, uncharged particles which impinge on the window 10 can causeadditional charging of the window 10 by a frictional mechanism. Chargedand uncharged particles can also give rise to charging of parts of theexterior of the aircraft other than the outer surface of the window 10.As the aircraft is flown through the aerosol, the window 10 becomespolarised as a result of an electric field generated by one or more ofthese mechanisms. This polarisation gives rise to an induced charge onthe interior of the window 10, and the electric field resulting fromthis induced charge is detected by the sensor 16. At each of a series oftimes, the output of the sensor 16 is recorded by the data acquisitionsystem 18 together with the position of the aircraft as determined bythe GPS 22.

The processor 24 is arranged to process data stored in the dataacquisition system 18. The processor 24 may be carried on the aircraftand arranged to process the data in real-time or it may be used toprocess data off-line, with data only being stored whilst the aircraftis in flight. The processor 24 is arranged to convert recorded values ofthe output of the sensor 16 to values of particles density on the basisof an assumed functional relationship between the electric field due toinduced charge on the interior of the window 10 (equivalent to theoutput of the static monitor 16) and particle density in the aerosolthough which the aircraft is flown. In some situations the relationshipmay be very simple, e.g. the electric field (and hence the output of thesensor 16) may be directly proportional to aerosol particle density. Inother cases the output of the sensor 16 may be a more complex functionof particle density. In still further cases the rate of change of theoutput of the sensor 16 may be a linear or a more complex function ofaerosol particle density. The functional relationship for a particulartype of aerosol may be guessed or found previously from experience usingother instruments or measurements. The processor 24 thus allows aerosolparticle density as a function of position to be found, i.e. aerosolparticle density to be mapped.

FIG. 3 shows a portion of a second example apparatus of the invention.Parts of the apparatus shown in FIG. 3 which correspond to parts of theapparatus shown in FIG. 1 are labelled with reference signs which differby 100 from those labelling the corresponding parts in FIG. 1. In thesecond example apparatus the output of an electro-mechanical field millsensor 116 is connected to a processor 117 which converts the output ofthe sensor 116 (or the rate of change of the output of sensor 116) inreal-time to a value of aerosol particle density on the basis of anassumed functional form for aerosol particle density as a function ofthe output of the sensor 116 (or the rate of change of the output of thesensor 116). Output from the processor 117 corresponds directly toaerosol particle density, which is recorded at each of a series of timesby a data acquisition system 118, together with the position of theaircraft as indicated by a GPS 122. The data acquisition system 118therefore stores information mapping aerosol particle density as afunction of position.

FIG. 4 shows a trace 210 of the output of an integrating nephelometermounted on a research aircraft taken over a four hour period duringwhich the aircraft was flown through a portion of the volcanic ash cloudproduced by the eruption of the Eyjafjallajökull volcano in Icelandwhich began on 20th March 2010. The trace 200 is referred to thevertical axis 211. The integrating nephelometer measures opticalextinction over three visible wavelengths and its output isrepresentative of aerosol particle density. FIG. 4 also shows a trace200 (referred to vertical axis 201) of the output of anelectro-mechanical field mill sensor over the same time period, thesensor being mounted within the same research aircraft in the mannerindicated in FIGS. 1 and 3. FIG. 4 shows that the output of theelectro-mechanical field mill was closely related to aerosol particledensity as indicated by the extinction measured by the nephelometer.

In FIG. 5, trace 220 (referred to vertical axis 221) is the same astrace 200 in FIG. 4. FIG. 5 also shows a trace 230 of the output of apassive cavity aerosol probe (PCASP), also mounted on the researchaircraft, during the same four hour time period during which the trace220 was recorded. (Trace 230 is referred to vertical axis 231). A PCASPis an optical spectrometer for detecting and analysing aerosols. FIG. 5shows a close correlation between aerosol particle density, as measuredby the PCASP, and the output of the electromechanical field mill sensormounted within the research aircraft.

FIG. 6 shows a trace 240 of the rate of change of the output of the sameelectromechanical field mill sensor over a period of 3.5 hours (referredto vertical axis 241) and also a trace 250 of the mass concentration ofvolcanic ash over the same period as determined by a dedicatedparticle-density measuring instrument fixed to the research aircraft.FIG. 6 shows a close correlation between the rate of change of theoutput of the sensor and the aerosol particle density of the volcanicash cloud through which the research aircraft was flown.

In some embodiments the output of the detection means may be monitored(e.g. input to a comparator) so that a warning signal may be generatedif the output exceeds a threshold level associated with a level ofaerosol particle density likely to damage the aircraft in some way (e.g.engine damage). The warning signal could be used to give a visual and/oraudible signal to the pilot of the aircraft. Alternatively, oradditionally, the warning signal may be used to automatically controlthe flight control systems of the aircraft so that the aircraft issteered to a region of airspace with a lower aerosol particle density.

1-15. (canceled)
 16. Aerosol detection apparatus comprising an aircrafthaving a dielectric member comprised in a body thereof such that asurface of the dielectric member forms part of the exterior surface ofthe aircraft, and a sensor located in the interior of the aircraft andarranged to detect an electric field within the aircraft, the electricfield resulting from the dielectric member becoming polarised so thatinduced charge appears on a surface of the dielectric member opposite tothat forming part of the exterior surface of the aircraft.
 17. Apparatusaccording to claim 16 wherein the dielectric member comprises a windowof the aircraft.
 18. Apparatus according to claim 16, wherein the sensorcomprises a static monitor mounted within the aircraft.
 19. Apparatusaccording to claim 18 wherein the static monitor comprises anelectro-static voltmeter.
 20. Apparatus according to claim 19 whereinthe electro-static voltmeter comprises an electro-mechanical field millinstrument.
 21. Apparatus according to claim 18, further comprising analarm configured to generate a signal if a monitored output of thestatic monitor exceeds a pre-determined threshold.
 22. Apparatusaccording to claim 21, wherein the signal comprises at least one of: avisual signal for the attention of one of more members of the aircrew,an audible signal for the attention of the one or more members of theaircrew, and a control signal to the aircraft control systems, causingthe aircraft to move out of a certain region of airspace.
 23. Apparatusaccording to claim 18, further comprising a data acquisition systemarranged to record one of the output of the static monitor and the rateof change of the output of the static monitor, as a function of theposition of the aircraft.
 24. Apparatus according to claim 23 whereinthe apparatus further comprises a global positioning system arranged tooutput positional information for the aircraft to the data acquisitionsystem.
 25. Apparatus according to claim 23, further comprising aprocessor configured to convert one of recorded values of the output ofthe static monitor, and recorded values of the rate of change of theoutput of the static monitor, to values of aerosol particle density onthe basis of an assumed functional form for aerosol particle density asa function of one of the output of the static monitor and the rate ofchange of the output of the static monitor.
 26. Apparatus according toclaim 18, further comprising a processor arranged to convert one of theoutput of the static monitor and the rate of change of the output of thestatic monitor to values of aerosol particle density on the basis of anassumed functional form for aerosol particle density as a function ofone of the output of the static monitor and the rate of change of theoutput of the static monitor.
 27. Apparatus according to claim 26further comprising a data acquisition system arranged to record valuesof aerosol particle density output by the processor as a function of theposition of the aircraft.
 28. Apparatus according to claim 27 furthercomprising a global positioning system arranged to output positionalinformation for the aircraft to the data acquisition system.
 29. Amethod of detecting particles in an aerosol comprising of causing theapparatus according to claim 16 to pass through a region of theatmosphere containing the particles.
 30. A method according to claim 29,wherein the region of the atmosphere contains dust or ash particles.